The present invention relates to a technique effective in application to a radio communication system having a phase control loop and an amplitude control loop for phase and amplitude modulation, and more particularly relates to a technique effective in application to a semiconductor integrated circuit for communication with a phase detection circuit and an amplitude detection circuit built in, and to a transmitter such as a cellular phone in which such a semiconductor integrated circuit for communication has been incorporated.
In the background art, as one of systems of transmitters (mobile transmitters) such as cellular phones and the like, there is a system called GSM (Global System for Mobile Communication) which has been adopted in Europe. In this GSM system, a phase modulation system called GMSK (Gaussian Minimum Shift Keying) is used as its modulation system so as to shift the phase of a carrier signal in accordance with transmitting data.
Generally, a high frequency power amplification circuit is incorporated in a transmission-side output portion in a transmitter. Some conventional transmitters using the GSM system adopt a configuration in which a bias voltage to the high frequency power amplification circuit is controlled to obtain output power required for a telephone call by a control voltage outputted from a circuit called an APC (Automatic Power Control) circuit generating a control signal for transmitting output on the basis of a signal from a detector detecting the transmitting output and a transmitting level requested from a base band LSI.
In recent cellular phones, an EDGE (Enhanced Data Rates for GSM Evolution) system has been proposed. The EDGE system has a dual-mode communication function in which communication of audio signals is performed in GMSK modulation while data communication is performed in 3 π/8 rotating 8-PSK (Phase Shift Keying) modulation. The 8-PSK modulation can be regarded as a modulation in which an amplitude shift is further added to a phase shift of a carrier signal in the GMSK modulation. While one-bit information is sent per symbol in the GMSK modulation, 3-bit information can be sent per symbol in the 8-PSK modulation. Thus, communication can be performed at a higher transmission rate in the EDGE system than in the GSM system.
As a method for implementing a modulation system in which the phase component and the amplitude component of a transmitting signal are allowed to carry information respectively, there has been heretofore known a system called a polar loop in which a signal to be transmitted is separated into a phase component and an amplitude component, fed back by a phase control loop and an amplitude control loop respectively, synthesized and outputted by an amplifier (for example, “High Linearity RF Amplifier Design” by Kenington, Peter B., p. 162, published by ARTECH HOUSE, INC., 1979).
In a communication system using the GSM system, it will go well if a phase-modulated signal is outputted in accordance with a requested output level. Accordingly, a radio frequency power amplification circuit in the final stage can be operated in the saturation region. On the other hand, in a radio communication system capable of performing transmission and reception using the EDGE system, a radio frequency power amplification circuit in the final stage has to be operated linearly in the non-saturation region because it is necessary to perform amplitude control. However, in a conventional system for driving a radio frequency power amplification circuit used in a communication system using the GSM system, it is difficult to secure linearity requested of the radio frequency power amplification circuit in an area of a low output level. On the other hand, according to the polar loop system, there is an advantage that the request for the linearity of the radio frequency power amplification circuit can be satisfied while the power efficiency in an area of a low output level can be also improved.
Therefore, the present inventors made investigations on adoption of the polar loop system in a radio communication system using the EDGE system. As a result, it was found that there was a problem that it was difficult to satisfy requested specifications in the error vector magnitude (EVM) of transmission waveform, the degree of noise suppression, and the like, when the 8-PSK modulation was implemented in the polar loop system.
Specifically, with a broader frequency bandwidth (the width between the central frequency of a transmitting carrier signal and a frequency in which the open loop gain reaches 0 dB) of the amplitude control loop, EVM is enhanced, and a characteristic called spectral regrowth showing the degree of waveform distortion is also improved. However, if the bandwidth is broader, there will arise a problem as follows. That is, the attenuation of the amplitude control loop in the reception frequency 20 MHz distant from the central frequency of the transmitting carrier signal becomes so small that a sufficient degree of noise suppression cannot be obtained.
However, of the two feedback loops, particularly the amplitude control loop has a loop gain varied due to scattering in manufacturing, so that the stability of the loop is degraded. It is therefore difficult to obtain a desired output level in a predetermined time. Further, in a system using the polar loop system, it was found that there was a problem that when the output control was carried out for the amplitude modulation in the amplitude control loop, the gain of the amplitude control loop varied so that the phase margin was reduced and hence the stability of the loop was degraded.
When a radio frequency IC for a transmitter capable of performing both the transmission using a GMSK modulation mode and the transmission using an 8-PSK modulation mode is configured, each of the feedback loops requires a loop filter for limiting the frequency bandwidth of the loop. It was, however, found that it was desired to change the frequency bandwidths of the respective loops between the GMSK modulation mode and the 8-PSK modulation mode. It was considered at first that a phase control loop for use in the GMSK modulation mode and a phase control loop for use in the 8-PSK modulation mode were provided separately. It was, however, found that there was a problem that such a configuration led to increase of the number of components for constituting the loop filters having different properties so that the circuit scale increased.
At the beginning of transmitting in the 8-PSK modulation mode, the two feedback loops, that is, the phase control loop and the amplitude control loop have to be first stabilized. In that case, however, it is difficult to stabilize the two feedback loops concurrently. It was therefore considered that the phase control loop was first stabilized and the amplitude control loop was then stabilized. It was, however, found that there was a problem that the two feedback loops had to be stabilized in a short time in the EDGE system while the GSM system had a comparatively large time margin because only the phase control loop had to be stabilized.
In the conventional GSM system, an APC circuit generating a control signal for transmitting output on the basis of a signal from the detector detecting the transmitting output and a transmitting level requested from the base band circuit has been used as described above. The present inventors hit upon an idea that if the amplitude control loop provided in the polar loop was used, the APC circuit could be omitted when the GMSK modulation mode was implemented in the GSM system or the EDGE system. The inventors therefore made investigations on the idea. As a result, it was found that there was a problem as follows. That is, when a loop filter on the amplitude control loop was designed to meet the GMSK modulation mode, the degree of noise suppression in the reception signal frequency band 20 MHz distant from the carrier frequency became insufficient at the time of operation in the 8-PSK modulation mode. On the contrary, when the loop filter was designed to meet the 8-PSK modulation mode, the phase margin of the loop became insufficient at the time of operation in the GMSK modulation mode or in the case where the output of the power amplifier was very small. Thus, the loop was instable, that is, apt to oscillate.